1. Field of the Invention
The present invention relates to a stretched multilayered film with excellent gas barrier properties that is formed by laminating a resin composition layer comprising an ethylene-vinyl alcohol copolymer (hereinafter referred to as xe2x80x9cEVOHxe2x80x9d) and a polypropylene layer via an adhesive resin layer to form a multilayered structure and stretching the multilayered structure in a high stretch ratio at least in one direction.
2. Description of the Prior Art
EVOH films have excellent transparency, barrier properties to oxygen, carbon dioxide, and nitrogen, and oil resistance. With such characteristics, the EVOH films are used as a packaging material for food, pharmaceutical products or the like. However, the EVOH films are moisture-sensitive and the barrier properties thereof deteriorate under high humidity. In addition, the EVOH films disadvantageously have a poor impact strength. In order to compensate for these shortcomings, an EVOH is used in the form of a laminate comprising an EVOH and polyolefin, which has excellent moisture-proof properties and impact properties, such as polypropylene.
However, the following problem is caused in the production of a film by laminating an EVOH and a polypropylene resin: Molding processes such as stretching can be performed easily for the polypropylene resin, whereas the EVOH has poor stretchability.
In the production of a film by laminating polyolefin and an EVOH, great ingenuity has been exerted to impart sufficient stretchability to the EVOH. For example, U.S. Pat. No. 4,561,920 (Japanese Laid-Open Patent Publication No. 60-187538) describes using an EVOH having a melt flow rate (hereinafter, referred to as xe2x80x9cMFRxe2x80x9d) of at least about 8 g/10 minutes, rapidly cooling a multilayered sheet comprising the EVOH so that the crystallinity of the EVOH is not more than about 25%, and then stretching the multilayered sheet.
Furthermore, another example of a multilayered barrier film comprising an EVOH is described in U.S. Pat. No. 4,880,706 (Japan Patent No. 2,679,823). This patent publication discloses a multilayered film obtained by the following method. First, a multilayered sheet having a structure of a polypropylene layer/an adhesive layer/an EVOH layer/an adhesive layer/a polypropylene layer is formed and immediately cooled rapidly to about 50xc2x0 C. Then, the thus-obtained sheet is preheated in the range from 135 to 150xc2x0 C. and stretched to 4 to 7 times larger in the longitudinal direction. Then, the sheet is preheated in the range from 170 to 180xc2x0 C. and stretched in the transverse direction in the range from 155 to 165xc2x0 C. Furthermore, this patent publication describes that it is preferable to use an EVOH having an ethylene content of at least 45 mol %, a degree of hydrolysis of at least 99%, and an MFR of 14 to 18 g/10 minutes at 190xc2x0 C. and a load of 2,160 g in order to be provided with stretchability. In addition, the publication states that it is preferable to use a polypropylene having an MFR of 2 to 6 g/10 minutes as the polyolefin that is laminated to the EVOH.
In stretching the multilayered sheet comprising the polypropylene and the EVOH as described above, it is preferable to allow the EVOH to have sufficient stretchability; namely, it is preferable that the EVOH has a high ethylene content. However, it is well known that the lower the ethylene content is, the better the gas barrier properties of the EVOH are, except when it is under extremely high humidity conditions such as 100% RH. Therefore, the gas barrier properties have been sacrificed for sufficient stretchability in a stretching process, in which an EVOH having an ethylene content of 45 mol % or more must be used.
On the other hand, Japanese Laid-Open Patent Publication No. 8-311276 discloses a multilayered film comprising an EVOH resin composition exhibiting a specific melting curve measured by a differential scanning calorimeter (DSC). Although the publication describes that this multilayered film can be stretched to 24 to 50 times greater, it actually was stretched to only 24 times greater. Thus, this publication fails to disclose a film that is stretched in a high stretch ratio and has a high modulus of elasticity at a high temperature. Furthermore, since the saponification degree of the EVOH used is relatively low, the EVOH is susceptible to heat deterioration when the production of the film is performed continuously over a long period.
Generally, a stretching operation in the transverse direction adapted for a multilayered sheet comprising a polypropylene layer is performed in the temperature range from a temperature in the vicinity of 150xc2x0 C. to a temperature in the vicinity of the melting point of the polypropylene, so that the stretched polypropylene film can exhibit sufficient mechanical properties. Therefore, in order to stretch the laminated sheet of the EVOH and polypropylene in a high stretch ratio, it is preferable that the temperature range in which the EVOH can be stretched is in the stretching temperature range for the polypropylene sheet, especially in the stretching temperature range in the transverse direction.
On the other hand, as the ethylene content of the EVOH is smaller, namely, as the melting point is higher, the EVOH can exhibit sufficient barrier properties, which is the largest characteristic of the EVOH. However, in the case where the melting point of the EVOH is excessively high, the laminated sheet comprising the EVOH and polypropylene cannot have sufficient stretchability, so that a satisfactory film cannot be obtained.
Therefore, the object of the present invention is to provide a multilayered film comprising an EVOH and a thermoplastic resin such as polypropylene that has a high stretch ratio, excellent gas barrier properties and a high modulus of elasticity even in a high temperature.
The present invention was accomplished by the inventors of the present invention, who made the research for an EVOH that has good barrier properties and can be stretched in a high stretch ratio when laminated to polypropylene, and found a suitable EVOH composition. Hereinafter, the present invention will be described more specifically.
A multilayered film of the present invention is formed by stretching a multilayered structure to 7 to 12 times larger at least in one direction. The multilayered structure is formed by laminating a resin composition (A) layer and a polypropylene (C) layer via an adhesive resin (B) layer. The resin composition (A) comprises two EVOHs (a1 and a2) having different melting points and satisfies the following formulae (1) to (3):
150xe2x89xa6MP(a1)xe2x89xa6172xe2x80x83xe2x80x83(1)
162xe2x89xa6MP(a2)xe2x89xa6180xe2x80x83xe2x80x83(2)
4xe2x89xa6{MP(a2)xe2x88x92MP(a1)}xe2x89xa630xe2x80x83xe2x80x83(3)
where MP (a1) represents the melting point (xc2x0 C.) of the EVOH (a1) measured by a differential scanning calorimeter (DSC), and MP(a2) represents the melting point (xc2x0 C.) of the EVOH (a2) measured by DSC.
In one preferable embodiment of the present invention, the ratio of the thickness of the resin composition (A) layer to the total thickness of the multilayered film is 3 to 30%, the ratio of the thickness of the adhesive resin (B) layer to the total thickness of the multilayered film is 1 to 30%, the ratio of the thickness of the polypropylene (C) layer to the total thickness of the multilayered film is 40 to 96%, and the dynamic modulus of elasticity (Exe2x80x2) of the multilayered film at 170xc2x0 C. in dynamic viscoelasticity measurement (under a load of 11 Hz sine wave) is 3xc3x97107 dyn/cm2or more.
In another preferable embodiment of the present invention, the resin composition (A) has an average ethylene content of 38 to 45 mol % and an average saponification degree of 99% or more.
In still another preferable embodiment of the present invention, the resin composition (A) comprises three EVOHs (a1, a2 and a3) having different melting points and satisfies the following formulae (4) to (6):
MP(a1) less than MP(a3) less than MP(a2)xe2x80x83xe2x80x83(4)
3xe2x89xa6{MP(a3)xe2x88x92MP(a1)}xe2x89xa620xe2x80x83xe2x80x83(5)
3xe2x89xa6{MP(a2)xe2x88x92MP(a3)}xe2x89xa620xe2x80x83xe2x80x83(6)
where MP(a1) represents the melting point (xc2x0 C.) of the EVOH (a1) measured by DSC, MP(a2) represents the melting point (xc2x0 C.) of the EVOH (a2) measured by DSC, and MP (a3) represents the melting point (xc2x0 C.) of the EVOH (a3) measured by DSC.
In yet another preferable embodiment of the present invention, the adhesive resin (B) is polypropylene modified with carboxylic acid.
In another preferable embodiment of the present invention, the total thickness of the multilayered film is 10 to 100 xcexcm, and the thickness of the resin composition (A) layer is 1 to 10 xcexcm.
In still another preferable embodiment of the present invention, the multilayered film is formed by stretching the multilayered structure at 140 to 200xc2x0 C.
In yet another preferable embodiment of the present invention, the multilayered film is formed by co-extruding the resin composition (A) layer, the polypropylene (C) layer and the adhesive resin (B) layer simultaneously to form a multilayered structure, and biaxially stretching the multilayered structure to 4 to 7 times larger in the longitudinal direction and 7 to 12 times larger in the transverse direction. In another preferable embodiment of the present invention, the multilayered film is formed by extrusion-coating the resin composition (A) layer on the polypropylene (C) layer that has been stretched to 4 to 7 times larger in the longitudinal direction to form a multilayered structure, and biaxially stretching the multilayered structure to 7 to 12 times larger in the transverse direction.
According to another aspect of the present invention, a multilayered film is formed by stretching a multilayered structure to 7 to 12 times larger at least in one direction. The multilayered structure is formed by laminating a resin composition (A) layer and a polypropylene (C) layer via an adhesive resin (B) layer. The resin composition (A) comprises of an EVOH and satisfies the following formulae (7) to (9):
115xe2x89xa6MSxe2x89xa6140xe2x80x83xe2x80x83(7)
180xe2x89xa6MExe2x89xa6195xe2x80x83xe2x80x83(8)
52xe2x89xa6(MExe2x88x92MS)xe2x89xa680xe2x80x83xe2x80x83(9)
where MS represents the melting start temperature (xc2x0 C.) of the resin composition (A) measured by DSC, and ME represents the melting end temperature (xc2x0 C.) of the resin composition (A) measured by DSC.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.